1. Field of the Invention
The invention relates to a control system for a wind power plant with a hydrodynamic gear in the drive train between wind rotor and a generator connected to the electric mains with fixed frequency.
2. Description of the Related Art
If attention is given to the special system characteristics of wind power plants with respect to the power input side first, there is a particularity, in addition to the progress over time of typically strongly fluctuating power potential of the air flow, in the power conversion of the mechanical power of the air flow into the mechanical power of the wind rotor. In this respect it is possible to associate each flow speed of the driving air flow with an optimal speed-to-torque ratio for the wind rotor, which on its part depends on the geometry and design of the wind rotor. The progress curve describing an optimal speed of the wind rotor, which depends on the wind speed, is known as parabolics. The term of an efficiency-optimal speed is used in the present application for a rotor speed following said parabolics.
The conversion of the mechanical power of a wind power plant into electric power by means of an electric generator leads to a further requirement placed on wind power plants because the connection of the electric generator to an electric interconnected network grid also requires the observance of a constant system frequency.
When coupling an electric generator to the mains network, a first solution is to provide the entire drive train of the wind power plant (and thus also the wind rotor) in a fixed-speed manner. Such fixed-speed wind power plants can be connected easily to the electric interconnected network grid by using asynchronous generators on the basis of slip caused by this principle. The constancy of the speed on the drive train is transmitted by the gear onto the wind rotor, so that the wind rotor does not run over its performance optimum at different wind speeds.
An especially disadvantageous aspect in fixed-speed wind power plants is that they can only be operated with reduced efficiency under partial load, which occurs frequently under typical wind conditions.
If a wind power plant is operated in general and in particular in the partial-load range with a variable rotor speed, there is either the possibility to provide a drive train with variable or constant initial speed, which leads to a variable or constant generator speed. The output also changes over time in both cases due to the time-varying moment.
The first case leads in wind power plants to the use of frequency converters, which excite the generator with the required frequency or provide compensations to the difference of the existing system frequency and thus realize a speed-variable generator. This approach leads away from the problem to be solved here and is linked to special problems such as the complex open-loop and closed-loop control circuits, the adversely reflected parabolic characteristics of the wind rotor in the frequency converter, the stiffness of the defined generator characteristics by the frequency converter, the lower operational reliability in the case of serious environmental restrictions, a mains supply quality to be operated with high effort such as the occurrence of only few harmonics and a low production of reactive power.
The second approach, namely combining a variable rotor speed of the wind power plant with a constant generator speed without the use of a frequency converter, corresponds to the topic represented herein of a wind power plant with variable input speed and constant output speed. The known solutions of this problem use a superposition gear, which branches the mechanical power. Only two approaches based on this have become known in connection with speed-variable wind power plants, which are used for keeping constant the generator frequency.
In the first system the input power via the superposition gear is divided among a large generator and a small servomotor, with approximately 30% of the input power being usually transmitted to the servomotor. The generator is connected with a fixed frequency to the electric network, whereas the servomotor is connected via a frequency converter to the mains network or is supplied via an auxiliary generator, which is mechanically coupled to the generator. For the purpose of stabilizing the generator speed, the servomotor is operated as a motor or as a generator with different frequencies. Such a system has the same problems as wind power plants with frequency-controlled generators.
In a second system, which works in a hydrostatic manner, hydraulic motors and pumps are used instead of the electric servomotor. This system also has the problem of difficult control characteristics, especially a sluggish response behavior and relevant idle times and strong non-linearities. Moreover, the hydraulic system components are disadvantageous due to the constructional complexity.
In addition to the above requirements placed on the control system of a wind power plant, which arise from an efficiency-optimal guidance of the speed and the use of a generator connected to the mains network, there are further requirements placed on control system, which arise from different operating ranges and operating states of a wind power plant. In the present application, the term of operating ranges is seen depending on the available wind and the degree of capacity utilization of the wind power plant. Three different, mutually adjacent operating ranges are distinguished below in an exemplary manner. They are designated by being sorted from low to high wind speeds as partial load range or parabolic load operation, speed-guided range for noise reduction and performance-limited full load range. The different operating states of a wind power plant must be distinguished from this. This can be the start phase, the synchronization phase, the stop phase or a braking of the wind power plant until standstill. Further operating states can arise from the requirements of mains network connection. This can concern a load rejection, a short circuit, a reactive power requirement or a power reduction.